Ethylene is an important basic bulk raw material in organic chemical industry, which is mainly used for producing polyethylene, polyvinyl chloride, ethylene oxide/ethylene glycol, ethylbenzene/styrene, vinyl acetate and other kinds of organic chemical products. Currently, ethylene is mainly produced through catalytic cracking method using petroleum as raw material at home and abroad. In recent years, the research on the production of ethylene using non-petroleum raw material has produced new breakthroughs, especially the technology of producing ethylene through ethanol dehydration. In this technology, bio-derived ethanol is used as raw material, so that the dependence on petroleum resources is avoided. For oil-poor countries and regions, as well as for countries and regions with agriculture as the main industry, the raw material for producing ethanol can be obtained easily and is reliable, which lays a solid raw material foundation for the production of ethylene, and the problems of fossil resources shortage and severe environmental pollution can be solved. Especially, with the rapid development of bio-technology, the technology of producing ethanol with biological methods has been improving continuously. The sources of raw material for producing ethanol are becoming more and more various, and the cost of raw material is becoming more reasonable, therefore, the technology of producing ethylene through ethanol dehydration is gaining more and more attentions.
It is important to develop a new technology with good economic benefits and strong market competitiveness for producing ethylene with ethanol, aiming at improving the technological process, reducing unit ethanol consumption and improving the efficiency of the apparatus.
There are many methods for producing ethylene through ethanol dehydration in documents and patents already disclosed at home and abroad, and the basic process thereof mainly comprises fixed-bed process and fluidized-bed process. It is raised by ABB Lummu in late 1970s to produce ethylene through ethanol dehydration with a fluidized-bed technology (U.S. Pat. No. 4,134,926), however, the technology has not been put into industrial applications. The fixed-bed process is mainly used in the present industrial applications, comprising isothermal fixed-bed process and adiabatic fixed-bed process.
Initially, ethanol dehydration reaction is conducted in tubular fixed-bed, the reaction pressure thereof is normal pressure, and the reaction heat is provided to the reaction by direct heating or indirect heating through heating medium (such as molten-salt). During the reaction process, the reaction temperature and the material flow speed are key factors. On the one hand, if the reaction temperature is too high or the material flow speed is too low, other by-products would be produced. On the other hand, if the material flow speed is increased, the conversion rate of ethanol would decrease. To solve the aforesaid problem, US patent (U.S. Pat. No. 4,232,179) raised the adiabatic process of ethanol dehydration reaction, i.e., ethanol dehydration reaction is conducted in adiabatic fixed-bed. The reaction material is heated to the required temperature before entering into the reactor, to ensure normal reaction. After that, a reaction technology of three-stage adiabatic fixed-bed (U.S. Pat. No. 4,396,789) is raised, and a ethylene production apparatus with a production capacity of 60 thousand tons per year is constructed using the aforesaid technology in the earlier 1980s. Three adiabatic fixed-bed reactors are connected in series in the technology, the mixed feedstock of ethanol and steam in each of the inlets of the reactors is pre-heated by one furnace, and the unreacted ethanol, ethyl ether and other by-products are recycled. The addition of steam reduces coking during reaction process, the lifetime of the catalyst is prolonged, and the yield of ethylene is improved. The operating data of the apparatus showed that, when the temperature of the inlet of the reactor is 450° C., the conversion rate of ethanol reaches 98%, and the regeneration period of the catalyst is at least one year. In addition, Halcon/SD Company developed a dual-mode technology of adiabatic or isothermal fixed-bed in the 1970s, wherein the isothermal fixed-bed technology was put into industrial application. The operating data of the apparatus showed that, under the conditions of the reaction temperature being 318° C. and the LHSV being 0.23 hour−1, the selectivity of ethylene is 96.8% (by molar), the conversion rate of ethanol is 99.1%, and the regeneration period of the catalyst is 8 months. After that, Halcon/SD Company developed the reaction technology of multi-stage adiabatic fixed-bed, i.e., ethanol, after being diluted in water steam, enters into the multi-stage adiabatic fixed-bed to produce ethylene through ethanol dehydration reaction; polymer grade ethylene product is obtained after the reaction gas is washed, compressed, alkaline cleaned, dried and cryogenic rectified. The test data showed that, under the conditions of the reaction temperature being 465° C., the LHSV being 0.8 hour−1, and the ratio of water steam and ethanol being 3:1, the selectivity of ethylene is 99.4% (by molar), the conversion rate of ethanol is 99.9%, and the regeneration period of the catalyst is 8 months. Lummus Company realized the industrialization of fixed-bed technology as early as 1960s. The tubular isothermal reactor was used in the technology, and the heat needed during the reaction process is provided by a hot oil system. The regeneration period of silica-alumina catalyst is 3 weeks, and it needs 3 days to regenerate the catalyst once. Under the conditions of the reaction temperature being 315° C. and the reaction pressure being 0.16 MPa, the selectivity of ethylene is 94% (by molar), and the conversion rate of ethanol is 99%.
The products separation technology and how to improve the yield of ethylene during the process of producing ethylene through ethanol dehydration are relatively less referred to in the prior documents or reports, especially the technical problem of recycling ethylene in light components streams and heavy components materials. Two-column process is mainly used in the prior separation technology, in which the unrefined ethylene enters into the ethylene fractionating column first, after the light components are condensed at the top of said column, the vapor-phase components are exhausted, the liquid-phase components are fed to the light components stripper, whereby the light components are removed from the liquid-phase components, and the refined ethylene with the purity of 99.99% is obtained. Although the refined ethylene with the purity of 99.99% is obtained by the aforesaid technology, since the stripper is used to remove the light components, and ethylene in the heavy components materials is not recycled, there is the problem of ethylene loss in the aforesaid technology. The patent (ZL200710040705.64) raised an adiabatic flash method to recycle ethylene from the liquid at the bottom of the ethylene fractionating column. Although ethylene is partly recycled with the aforesaid method, the recycling efficiency thereof is relatively low. The present disclosure provides a new separating and refining method for producing ethylene through bio-derived ethanol dehydration, with the advantages of high ethylene recycling efficiency, low power consumption and good product quality.
In addition, the processing technology of organic wastewater in producing ethylene through ethanol dehydration is relatively less referred to in the prior documents or reports, especially how to reduce the consumption of the raw material, i.e., ethanol, and the technical problem of resource utilization of organic wastewater. Currently, the organic wastewater of industrial apparatus directly enters into the wastewater treatment equipment out of said apparatus without being treated. Consequently, on the one hand, the organic substances, especially ethanol in the organic wastewater are not utilized, and the consumption of ethanol increases; on the other hand, the concentration of organic substances in wastewater is relatively high, as a result, the wastewater treatment difficulty and treatment cost of wastewater treatment equipment out of said apparatus increase inevitably. The patent (CN101376551B) raised a treatment method for treating organic wastewater during the production of ethylene through ethanol dehydration with a four-unit process, but there are the disadvantages of complicated technology, long process and high investment in the aforesaid method.